The Predatory Bird Research Group (PBRG), University of California, Santa Cruz, is conducting a long-term field study of the ecology of golden eagles (Aquila chrysaetos) in the vicinity of the Altamont Pass Wind Resource Area (WRA). The facility lies just east of San Francisco Bay in California and contains about 6,500 wind turbines on 190 km2 of rolling grassland. Each year, the wind industry reports 28–43 turbine blade strike casualties of golden eagles in the WRA, and many more carcasses doubtless go unnoticed. Because golden eagles are naturally slow to mature and reproduce, their populations are sensitive to changes in adult and subadult survival rates. The U.S. Fish and Wildlife Service and the California Department of Fish and Game have therefore expressed concern that the fatalities might have an adverse effect on the population. PBRG’s four-year investigation of the population trend (January 1994 through December 1997) was supported for the first three months by the wind industry and thereafter by the National Renewable Energy Laboratory.

Annual nest surveys have revealed a substantial breeding population, the density of which is among the highest reported for the species. An 820–km2 area near the town of Livermore held at least 44 pairs in 1997, a density of one pair per 19 km2 . PBRG has estimated that at least 70 active territories exist within 30 km of the WRA boundary. Territory occupancy from year to year has been 100%, and the reproductive rate, based on an annual sample of about 60 pairs, averaged 0.61 fledged young (∼0.25 females) per occupied site.

To estimate survival rates, we tagged 179 eagles with radio transmitters equipped with mortality sensors and expected to function for at least four years. Population life stages represented in the tagged sample included 79 juveniles, 45 subadults, 17 floaters (non-territorial adults), and 38 breeders. Effective sample sizes in the older stages increased as eagles matured or became territorial. Thus, by the end of the study, we had obtained telemetry data on 106 subadults, 40 floaters, and 43 breeders, in addition to the 79 juveniles.

Weather permitting, we conducted weekly roll-call surveys by airplane to locate the radio-tagged eagles and to monitor their survival. The surveyed area, defined by the movements of tagged birds during the first few months of the study, extended from the Oakland Hills southeast through the Diablo Mountain Range to San Luis Reservoir about 75 km southeast of the WRA.

Of 61 recorded deaths of radio-tagged eagles during the four-year investigation, 33 (54%) resulted from electrical generation or transmission. Of these, 23 (38%) were caused by wind turbine blade strikes, and 10 (16%) by electrocutions on distribution lines, all outside the WRA. Additional fatalities went unrecorded because turbine blade strikes destroyed the transmitter in an estimated 30% of cases. The aerial surveys showed that breeding eagles rarely entered the WRA, whereas nonterritorial eagles tended to move about freely throughout the study area, often visiting the WRA.

Computer analysis of survival data (Program MARK) by Alan Franklin, Tanya Shenk, and Ken Wilson (1998) from Colorado State University considered Kaplan-Meier survival estimates among the various groupings of life stages and sexes. Their most parsimonious solution was a pooling of data from juveniles, subadults, and floaters of both sexes to produce a single estimate of annual survival for non-territorial eagles at 0.7867 (SE=0.0263). The estimate for the annual survival of territorial eagles (breeders) was 0.8964 (SE=0.0371).

Franklin, Wilson, and Shenk (1998) developed two Leslie matrix models to estimate the trend of the population. The first, which incorporates the rate at which non-territorial eagles become breeders, estimated the annual rate of population change (λ) at 0.9068 (SE=0.03). The 95% confidence interval of this estimate did not include λ = 1.0, the value for a stable population. This means that, if their model and its assumptions are valid, the population was in a state of decline during the period of our study.

The second model, configured at our request, estimated potential growth rate on the assumption that all maturing eagles enter the breeding segment. Part of our rationale was that, once a declining population loses its floating segment, the floater-to-breeder transition rate is moot and only adds variance to the trend estimate. This was of particular concern because the available floater-to-breeder transition rate estimate lacked precision (CV=66.7%). Moreover, the floater-to-breeder transition rate can be expected to change with population size and therefore cannot be modeled as a constant. Franklin, Wilson, and Shenk’s (1998) estimate of λ in the second (potential growth rate) model was 0.9880, a value statistically indistinguishable from unity. A Moffat life table model developed by Hunt (1998) yielded a virtually identical value for λ. Sensitivity analyses for both the matrix and Moffat models found the population most responsive to changes in adult survival and least affected by variation in juvenile survival and reproduction.

Several biological considerations suggest that the potential growth rate of the population is actually lower than estimated. First, we are likely overoptimistic in assuming perfect efficiency by nonterritorial eagles in filling breeding vacancies by the next breeding season. Second, eagles newly acquiring territories would be initially less fecund than those being replaced, reducing net population productivity. Third, true survival rates are likely lower than estimated because a proportion of transmitters were destroyed by turbine blades.

On the other hand, several factors may operate in favor of population persistence. If floaters immigrating from other subpopulations are available, they may buffer the breeding segment against decline. Moreover, average territory quality— and hence average per capita reproduction— can be expected to increase if the number of territories declines. Other points of optimism include the observed 100% annual territorial reoccupancy rate and the low incidence (3%) of subadults as members of breeding pairs, an indication that a reserve of floaters continues to exist.

The wind industry at Altamont Pass has recently initiated a number of measures that may reduce the rate of turbine blade strikes. These include modification of existing turbines, the removal of turbines in “high-risk” areas, and the replacement of turbine models with others thought to be more benign. In the latter case, the replacements are more efficient, the net result being far fewer turbines. To track the efficacy of these and other possibly mitigating changes, PBRG will continue to radio-tag eagles, monitor eagle movements and survival, conduct an annual nest survey, and model the accruing data to reassess the population trend.